New URLs: If you add domain mapping to an existing blog with posts, their URL’s will change. We’ll redirect any visitors looking for the old URL’s over to the new ones, so none of your visitors will get lost out there on the internet, but other services won’t always understand that the old links and the new links point to the “same” place. For example, Tweet counter widgets work based on particular URL’s, so old posts’ counts will be reset to 0. Once you make the change, future posts will not be affected. http://en.support.wordpress.com/domain-mapping/#important-notes-before-upgrading

I also want to move to a domain name from my current wordpress.com blog, I want to buy an upgrade. I am worried about my old posts, will they be moved to the new domain??
Also do I have to buy a domain redirect ??
Please let me know, I am eagerly waiting for the answer/resolution!